Thermal dye sublimation printing is a printing method in which dye from the dye donor element is heat transferred to a receiver sheet selectively in accordance with electrical pulsing of heating resistors in the thermal print head. The print head contains a plurality of resistors, so that the amount of dye transferred is selectively controlled by the intensity and duration of the resistor heating cycle.
The donor sheet includes a very thin support, usually polyester, one side of which is covered with a dye layer carrying the printing dyes.
Because the thin support softens when heated during the printing process and then sticks to the print head causing malfunction and degradation in the printing process, a heat resistant "slip" layer is added to the back of the support. The slip layer usually contains a lubricant and binder.
The slip layers used today consist of polymer layers, which in some cases contain particles of silica, alumina, talc, etc. These layers have poor resistance to continued use due to their thinness. Moreover, they also provide a thermal barrier to heat being transferred across the nip from the print head to the dye layer. It would be desirable that the slip layer provide a wear resistant sliding surface layer which is both thin (on the order of 3 to 10 .mu.") and is heat conductive.
The formation of thin durable wear resistant coatings has been described in the patent literature. U.S. Pat. No. 3,353,166 describes a plated surface of NiCo, a thickness of 6 .mu.", which was subjected to a heat treatment of 2 hours at a temperature of 700.degree. F. As a result of this treatment, an oxidized layer of Ni and Co was formed on the surface. The layer was formed in-situ and there was a gradual transformation of the surface from a metal to an oxide, with no distinct separating layer interface.
U.S. Pat. No. 3,460,968 discloses a method of forming an oxide wear resisting layer consisting of Co.sub.3 Cr.sub.4 on a magnetic layer by placing a high coercivity film in a temperature and humidity chamber until an oxide layer was formed. Humidity level was critical to avoid condensation onto the film.
U.S. Pat. No. 4,554,217 describes the in-situ formation of an oxide on the surface of sputtered or electron beam deposited CoCr, and NiCo by injecting a small amount of O.sub.2 into the deposition chamber in the latter stage of deposition. A base film of PET was used in the investigation which was subsequently tested as a floppy disk. Other U.S. Pat. Nos. 4,345,909; 4,399,013; 4,323,629; 4,124,736; 4,268,369; 3,498,837; 4,390,562; 4,390,601; and 4,411,963 address the issue of wear coat formation on flexible surfaces. The Sony Corporation produces a product under the trade name "High 8" consisting of a flexible media with a vacuum deposited coating, which has a layer of oxide formed in-situ during the coating process.
U.S. Pat. No. 4,124,736 describes a metal substrate having a magnetic layer coated with a barrier layer, which is in turn coated with an oxide layer. Cobalt oxide (presumably Co.sub.3 O.sub.4) is preferred due to its desirable friction resistance. U.S. Pat. No. 4,268,369 describes the use of SiO.sub.2 as an overcoat material on rigid disks. U.S. Pat. No. 4,390,562 describes a disk substrate with a magnetic coating, a magnetic layer, a converted metal into a metal oxide layer, and a rubbed on solid lubricant (namely a fluorocarbon).
None of these patent disclose the use of a heat conductive wear resistant layer as a slip layer for a reusable thermal dye donor element.
There is thus a need for providing in a reusable thermal dye donor belt, a multi-pass slip layer having enhanced wear resistance, while providing good heat conductivity from the thermal head to the dye layers.